International Journal of Sciences: Basic and Applied Research (IJSBAR) ISSN 2307-4531 (Print & Online) http://gssrr.org/index.php?journal=JournalOfBasicAndApplied --------------------------------------------------------------------------------------------------------------------------- Variability of Surface Chlorophyll-a in the Makassar Strait – Java Sea, Indonesia Muhammad Syahdana*, Agus S. Atmadipoerab, Setyo Budi Susiloc, Jonson L. d Gaol aFaculty of Fisheries and Marine Sciences, Lambung Mangkurat University, Banjarbaru, 70721, Indonesia b,c,dFaculty of Fisheries and Marine Sciences, Bogor Agricultural University, Bogor, 16680, Indonesia aEmail: [email protected] bEmail: [email protected] cEmail: [email protected] dEmail: [email protected] Abstract Eleven years (2002-2012) of surface chlorophyll-a (chl-a) data from the Aqua-MODIS satellite were analyzed to determine spatial pattern and temporal variation in the key region for the small pelagic fishery between the Makassar Strait and the Java Sea Indonesia. The surface chl-a time-series data can be explained by the first two eigenvectors mode, which accounted for about 90% of total variance. The first eigenvector mode (86%) displays a map of contours that are maxima in the northern Java and western Sulawesi, and are minima around Kalimantan. The corresponding principal component is mostly the annual fluctuation of chl-a with a maximum (minimum) during the peak of the northwest monsoon (the southeast monsoon) periods. There is a phase-lag of about 6-month of chl-a maxima between the northern Makassar Strait and the western Java Sea. On inter- annual time-scale fluctuation of chl-a is well correlated with the El Nino Southern Oscillation variability in the Pacific Ocean, as well as, the Indian Ocean Dipole Mode. Keywords: surface chlorophyll-a; Aqua-MODIS satellite; EOF analysis; Makassar Strait; Java Sea Indonesia. ------------------------------------------------------------------------ * Corresponding author. E-mail address: [email protected]. 103 International Journal of Sciences: Basic and Applied Research (IJSBAR) (2014) Volume 14, No 2, pp 103-116 1. Introduction Chlorophyll-a is an important pigment contained in phytoplankton for photosynthesis process, thus it becomes an indicator of the marine primary productivity level. The fluctuation of chlorophyll-a in the ocean is strongly influenced by oceanographic factors such as currents, seawater temperature, salinity, and nutrient. In addition, the nutrient content of the waters is closely related to the chlorophyll-a concentration in which the more the nutrient content of the water the higher the chlorophyll-a concentration [1]. The Makassar Strait waters exhibits much higher primary productivity than the others waters in Indonesia [2,3], which occurs throughout the year, both in the northwest monsoon (NWM) and in the southeast monsoon (SEM) periods. During the NWM, the high level of primary productivity is due to the runoff from the mainland of Kalimantan and Sulawesi in large numbers due to the high precipitation rate. While during the SEM, high primary productivity occurs due to the seasonal upwelling in the southern tip of Makassar [3]. The chlorophyll-a (chl-a) concentration in the Java Sea reported by [4] showed that in the western part of Java Sea, distribution of chl-a is relatively similar in both seasons, but in the eastern Java Sea the chl-a concentration increases during the SEM. Besides the seasonal fluctuations, inter-annual climate variability of ENSO and perhaps the Indian Ocean Dipole Mode (IODM) also affects the chl-a concentrations. During the El Nino year, sea surface temperature (SST) indicates negative anomaly and SST decreases to about 25.3 °C, whereas in the eastern Java Sea occurs positive anomaly of chl-a concentration [4]. This study attempts to investigate the spatial pattern and temporal variation of chl-a concentration between Makassar Strait and Java Sea. This region is well known as a fishing ground in a large-scale fishery, primarily by the purse seine fleet [5,6]. It is expected that this study could contribute useful information for fishery management purposes. 2. Materials and Methods The study area is located in the Makassar Strait and in the Java, Sea Indonesia (Fig. 1). The surface chl-a data have been acquired by the Aqua-MODIS satellite imagery Level-3 with the spatial resolution of 0.05° x 0.05° and temporal resolution of 8-day and time coverage from July 2002 to December 2012 (or about 11 years). The data were obtained from the website of the Pacific Islands Fisheries Science Center (PIFSC) which was part of the National Oceanic and Atmospheric Administration (NOAA) - USA [7]. The OC3M algorithms, developed by O 'Reilly, et.al. 2000 vide [8], has been implemented to estimate the chl-a concentration, using by the following equation : ( + + 2+ 3+ 4) = 10 0 1 2 3 4 = 0.2830, 2.753, 1.457, 0.659, 1.403 − − where, 104 International Journal of Sciences: Basic and Applied Research (IJSBAR) (2014) Volume 14, No 2, pp 103-116 443 > 488 = 10 551 � � Rrs is remote sensing reflectance Fig. 1. Map of the study area in Makassar Strait and Java Sea. Arrows show a schematic flow of the Indonesian Throughflow via Makassar. Dashed-line arrows are for surface monsoonal currents in the Java Sea. Letters (A- E) denote sampling box for time series (A=Northern Makassar Strait, B=Southern Makassar Strait, C=Southern South Sulawesi, D=Eastern Java Sea, E=Western Java Sea) The chl-a time-series data between 2002 and 2012 were analyzed using the Empirical Orthogonal Function (EOF) methods to extract a relatively small number of independent variables which convey as much of the original information as possible without redundancy [9]. The formula for the EOF analysis as shown below : ( , ) = ( )u ( ) =1 � X (t, s) is a function of space and time, where M is the number of modes or modes-M of decomposition to the signal space and time with the function uk space (s) and time functions ck (t). The time function in the EOF is also known as the expansion coefficient or principal component (PC) To understand the dominant period of variability within the surface chl-a time-series data in the study area, five sampling boxes were chosen within the Makassar and Java Sea (Fig. 1). The Continuous Wavelet Transform (CWT) was applied to analysis the signal in the chl-a time series data, which produces an instantaneous estimate or local value for the amplitude and phase of each harmonic. This allows detailed study of non-stationary spatial or time-independent signal characteristics [10]. 105 International Journal of Sciences: Basic and Applied Research (IJSBAR) (2014) Volume 14, No 2, pp 103-116 Following [10], the CWT method is formulated as follows : 1 ( , ) ( ) = − , ,=0 − � ∗ � � Assume that one has a time series, xn, with equal time spacing dt and n = 0 … N - 1. Also assume that one has a wavelet function,y, that depends on a nondimensional “time” .To be “admissible” as a wavelet, this function must have zero mean and be localized in both time and frequency space. The (*) indicates the complex conjugate. By varying the wavelet scale s and translating along the localized time index n, one can construct a picture showing both the amplitude of any features versus the scale and how this amplitude varies with time The wavelet transform can be used as a band-pass filter of uniform shape and varying location and width, as described in detail in [10]. Here, the low-frequency band-pass filter of chl-a time-series data on semi-annual, annual, and inter-annual were applied. Reconstructed new time-series of chl-a on these time-scales period are analyzed and compared within 5 sampling boxes. 3. Results and Discussion 3.1. Mean and Standard Deviation of Surface Chl-a Direct speaking, it is shown that distribution of high chl-a concentration (above 1 mg/m3) appears around the coastal area of Kalimantan (Fig. 2). Previous studies showed that total suspended matter may contribute significantly to the “high chl-a” concentration around Kalimantan [11]. Distribution of low chl-a content of about 0.1 - 0.2 mg/m3 is observed in the northern part of the Makassar Strait closer to the west coast of Sulawesi. This means that the northern entrance of Makassar is low chl-a content, which occurs throughout years. Fig. 2. Mean (2002-2012) (left) and standard deviation (right) of chl-a concentration (mg/m3) in the study area The Java Sea and the northern part of Makassar Strait (closer to the coast of the Kalimantan) shows moderate chl-a content of 0.3 to 0.4 mg/m3 with standard deviation of about 0.15. The southern tip of the Sulawesi exhibits high mean and standard deviation of chl-a concentration of 0.3 to 0.85 mg/m3 and of 0.15 to 0.45, 106 International Journal of Sciences: Basic and Applied Research (IJSBAR) (2014) Volume 14, No 2, pp 103-116 respectively. This implies that high mean chl-a concentration in this region is followed by high variation over the time. Low mean and standard deviation of chl-a content is visible in the southeastern part of the study area. 3.2. The Spatial Pattern and Temporal Fluctuation of Chl-a Concentration Fig. 3 shows the results from the EOF analysis for the chl-a time-series data. The first two EOF modes explained 90% of total variance. The first eigenvector (EOF1 mode) (86% of explained variance) displays a map of contours that are higher coefficients in the northern Java and western Sulawesi, and lower coefficients around Kalimantan (Fig. 3 left column). The corresponding principal component (PC) is an oscillation of chl-a with annual period. The amplitude of chl-a is maximum during the northwest monsoon period, in contrast to the chl-a minimum during the southeast monsoon (Fig.